less mixin bg-image problem

[whatwg6]

less syntax

.bg-image('./img/brand'); //error This relative module was not found:* ../../../static/css/img/brand@2x.png 

mixin.less

.bg-image (@url) {
  background-image: url("@{url}@2x.png");
}

i modify the path

.bg-image('../../src/components/header/img/brand');// success

the forum answer can slove this problem ,but why

.bg-image('./img/brand');// success

mixin.less

.bg-image (@url) {
  background-image: ~"url(@{url}@2x.png)";
}